This invention relates to an improved solid phosphoric acid catalyst composition having a total silicon phosphate X-ray intensity greater than 35.0 percent relative to alpha-alumina having been prepared by specific calcination conditions.
Solid phosphoric acid is a catalyst well known for its usefulness in various hydrocarbon conversion processes such as aromatic alkylation and olefin polymerization. The catalyst is composed of a support or substrate portion onto which is incorporated an acid fraction for catalytic activity. It is believed that the substrate portion is formed from the silica-phosphoric acid reaction, principally silicon orthophosphate, Si.sub.3 (PO.sub.4).sub.4, silicon pyrophosphate, SiP.sub.2 O.sub.7, as well as derivatives of these compounds. The catalyst is typically prepared by mixing silica with phosphoric acid followed by extrusion and calcination. The reactions are simply illustrated as follows: EQU 3SiO.sub.2 +4H.sub.3 PO.sub.4 .fwdarw.Si.sub.3 (PO.sub.4).sub.4 +6H.sub.2 O EQU SiO.sub.2 +2H.sub.3 PO.sub.4 .fwdarw.SiP.sub.2 O.sub.7 +3H.sub.2 O
The above reactions indicate that the phosphoric acid will react with silica to yield both types of phosphates depending upon stoichiometry and reaction conditions. The silicon orthophosphate can also be dehydrated during drying to give the silicon pyrophosphate, and this is believed to be the alternative mechanism for the silicon pyrophosphate formation. The silicon ortho- to pyro-phosphate conversion also depends on factors such as temperature and hydration, as illustrated by the following equations: EQU Si.sub.3 (PO.sub.4).sub.4 +2H.sub.3 PO.sub.4 .fwdarw.3SiP.sub.2 O.sub.7 +3H.sub.2 O EQU Si.sub.3 (PO.sub.4).sub.4 +heat.fwdarw.2SiP.sub.2 O.sub.7 +SiO.sub.2
These reactions have made the catalyst manufacturing more complex. Low activity or low stability catalysts have resulted due to low crystallinity caused by poor crystallization conditions.